This invention is generally related to the purification of aromatic dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid. More particularly, this invention is related to a practical method of reducing potassium in the final product to acceptable levels for subsequent polymerization.
Polymer grade aromatic dicarboxylic acids are important as starting materials for a number of polyester fibers, polyester films, resins for bottles and containers, and the like. Naphthalene dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid (hereafter referred to as 2,6-NDA) are starting materials for polyethylene naphthalates, which can also be employed in the manufacture of fibers, films, and resins. Presently in the art, it is very difficult to produce polymer grade 2,6-NDA and only dimethyl 2,6-naphthalene dicarboxylate, the methyl ester of the more desirable 2,6-NDA is commercially available for use in making polymers such as polyethylene naphthalate.
When aromatic dicarboxylic acids are produced by a disproportionation reaction, such as described in U.S. Pat. No. 2,823,231 and U.S. Pat. No. 2,849,482, for example, where an alkali metal salt of a mono- or dicarboxylic acid is disproportionated to produce isomers of the salt of the desired dicarboxylic acids, the product often retains undesirable amounts of the alkali metal, most often potassium.
The goal of a number of processes for purification of aromatic dicarboxylic acids, and 2, 6-naphthalene dicarboxylic acid in particular, is to reduce these alkali metals along with other impurities to the lowest levels possible, but there is still a need in the art for methods of reducing impurities and alkali metals to levels acceptable for use in polymers.
Methods of purification of aromatic dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid are known in the art. Where 2,6-NDA is produced by disproportionation, common methods for purification include filtration, acidification and crystallization. See, for example, U.S. Pat. Nos. 2,849,482; 3,631,096; 3,671,578; and 3,952,052. It is possible to remove significant amounts of color bodies and impurities, but it is still difficult to obtain polymer grade 2,6-NDA.
Currently in the art the most common process for making 2,6 NDA starts with relatively expensive o-xylene and butadiene feedstocks, as discussed, for example, in U.S. Pat. No. 5,510,563 and U.S. Pat. No. 5,329,058 and incurs substantial yield losses of these starting materials. Following the synthesis and purification of 2,6 dimethylnaphthalene (2,6 DMN), 2,6 DMN is oxidized to produce crude NDA product which forms as a solid with impurities trapped within. Therefore, in such processes, esterification to naphthalene dicarboxylate (NDC) is necessary to eliminate the impurities, as discussed in U.S. Pat. Nos. 5,254,719 and 4,886,901. Direct purification of the crude NDA via hydrogenation has been suggested by U.S. Pat. No. 5,292,934, but requires a difficult and expensive high temperature hydrogenation in the presence of a solvent. Another proposed purification scheme requires the use of nitrogen containing species. (See U.S. Pat. No. 5,770,764 and U.S. Pat. No. 5,859,294). Crystal size and morphology is important in either case, whereas the novel process disclosed herein can optionally avoid the issue of controlling particle size of the final product.
Currently NDC is commercially available, but NDA is not, presumably because of the difficulty of producing polymerization grade NDA without esterifying to NDC. Ideally, if NDA were available commercially at a competitive price, NDA would be preferred over NDC as the starting monomer for PEN. Alternative routes to NDA based on the rearrangement reaction have been plagued with difficulties associated with handling solids, the inefficient recycling of potassium, and ineffective integration from feedstock through final product. Although various improvements have been suggested over the years, there is still a distinct need in the art for an economical, integrated process for producing polymer grade 2,6-NDA, the preferred monomer for the production of polyethylenenaphthalate (PEN). Copending U.S. application Ser. No. 60/151,577 (Attorney""s Docket #TH1432) discloses a novel integrated process for producing the preferred 2,6-naphthalene dicarboxylic acid.
It would represent a distinct advance in the art if a method were available to reduce the levels of alkali metals, particularly potassium in product 2,6-NDA to levels which are acceptable for polymerization to polyethylene naphthalate.
In accordance with the foregoing, the present invention provides a method for reducing alkali metals in aromatic dicarboxylic acids produced by disproportionation or rearrangement of an alkali salt of a monocarboxylic acid to levels acceptable for polymerization which comprises:
a) Washing the aromatic dicarboxylic acid with water in a ratio of about 5:1, water to acid, at a temperature of about 70-200xc2x0 C.,
b) Introducing the washed aromatic dicarboxylic acid into a reactor characterized by minimal backmixing, such as a pipe reactor, and reacting the washed aromatic dicarboxylic acid in said reactor in approximately a 5:1 water to acid ratio at about 100-200xc2x0 C.;
c) Directing the aromatic dicarboxylic acid exiting the pipe reactor to a centrifuge to separate the solid aromatic dicarboxylic acid from water containing contaminants;
d) Optionally, combining the solid aromatic dicarboxylic acid again with water in a ratio of about 5:1 at a temperature of about 100-200xc2x0 C. to further reduce levels of alkali metals.
In the preferred embodiment, the invention makes it possible to reduce the level of potassium in product 2,6-naphthalene dicarboxylic acid to less than 50 ppm.
In the preferred embodiment the aromatic dicarboxylic acid which is treated in the present invention is 2,6-NDA produced by a process which incorporates disproportionation of an alkali metal salt of a mono- or dicarboxylic acid.
Following disproportionation the crude product is in the form of an isomer of a dialkali salt of 2,6 -NDA. Salts formed by the reaction can be transformed into the corresponding acids by acidifying the solution with acids or by introducing carbon dioxide into the solution and then separating the free acids from the acidified solution. The salt mixture produced by the reaction may also be transformed directly into derivatives of the acids, such as, for example, their esters or halides, and these derivatives can be purified, if desired, by fractional distillation.
The product treated in the present invention can be produced by the process described in copending Ser. No. 60/151,577 (Attorney""s Docket #1432), incorporated by reference herein in its entirety. In that process, after disproportionation the solid product consisting of K2NDA isomers is washed and the liquid is filtered to remove catalyst and coke particles.
The liquid carrying mixed organic salts is introduced into a two-stage evaporative crystallization section where the K2NDA is selectively precipitated, KHCO3 is recycled, and the purified K2NDA is redissolved with additional H2O. Then the purified K2NDA is passed through an activated carbon bed.
Next, the dipotassium salt of 2,6-NDA, (K2NDA) is selectively precipitated using CO2 to make the KHNDA solids which are then disproportionated into 2,6 -NDA and K2NDA. The product of disproportionation is centrifuged to yield a 2,6 NDA slurry, and a centrate containing predominantly 2,6 K2NDA and KHCO3 . The product 2,6-NDA contains from about 60-1000 ppm potassium on a dry basis. It is desirable to reduce the amount of potassium to 50 ppm or less.
The key to obtaining polymer grade product with  less than 50 ppm potassium without using excessive water is the combined use of a 5:1 water wash and a pipeline reactor to drive the reaction to completion and/or remove trace K+.
The 2,6-NDA and trace KHNDA are directed into a water wash. In the water wash a ratio of about 3 to 8 parts water is added to one part 2,6-NDA solid and KHNDA and reacted at a temperature of about 90 to 180xc2x0 C., for up to about one hour. Good results were observed washing with about 5 parts water and heating to about 150xc2x0 C. for about 30 minutes. The higher temperature is preferred because it affords higher solids loading.
The 2,6-NDA and KHNDA are then introduced into a reactor of the type known as a pipeline reactor, characterized by plug flow kinetics. In a plug flow reactor very little, if any, backmixing of product with feed occurs, as contrasted with a stirred vessel reactor or pipe loop reactor. Reactors of the type known in the art as turbulent flow would also be effective.
The temperature in the pipeline reactor should be in the range of 100-200xc2x0 C. The preferred range is about 140-170xc2x0 C., with a temperature in the range of 150xc2x0 C. providing good results.
The residence time in the pipeline reactor can vary. The slurry is reacted at plug flow conditions to drive the KHNDA disproportionation reaction toward completion and to remove trace potassium. In example 1 it was found that the desired results were achieved under plug flow conditions in about 30 minutes to an hour.
The contents of the pipeline reactor is directed to a centrifuge where trace contaminants are removed and recycled back to the KHNDA disproportionation reactor.
A second water wash is optional, but is included in the preferred embodiment. This second wash makes it possible to reduce the ppms of potassium to less than 50 ppm. The 2,6-NDA is combined with water, again at a ratio of about 3-8 parts water to one part 2,6-NDA and a temperature of about 100xc2x0 to 200xc2x0 C. Good results were achieved using about a five fold excess water, and increasing the temperature to about 1500C. Example 2 demonstrates the reduction of potassium levels to  less than 50 ppm.
The slurry containing the product 2,6-NDA is directed to a last centrifuge to separate the product from the water.
The 2,6 -NDA solid can be dried by conventional means, apparent to those skilled in the art, or as described, for example, in U.S. Pat. No. 5,292,934 or U.S. Pat. No. 5,840,968, incorporated by reference herein. However, where dry handling of the solid product is practiced, particle size control can be critical and the handling of 2,6- NDA particles is difficult and costly.
As mentioned above, it has previously not been possible in the art to obtain commercially available 2,6-NDA which is acceptable for polymerization. With the present invention it is now possible to produce 2,6-NDA with less than 50 ppm potassium. Other impurities are not as much of a problem as in the prior art either using the novel integrated process of copending Ser. No. 60/151,577 (Docket #TH1432) which avoids the isolation of purified naphthoic acid using a novel hydrodebromination step.
The process of copending Ser. No. 60/151,577 (Docket #1432) makes it possible to spring impurities. Therefore, it is now possible to produce 2,6-NDA with a low level of impurities and less than 50 ppm potassium. 2,6 -NDA of this quality can also be transported in a water slurry which is convenient for close coupling with a process for making polyethylene naphthalate (PEN), thus avoiding the difficulties associated with product particle size control, drying, and solids handling. This is discussed in more detail in U.S. Ser. No. 60/151,603(Attorney""s Docket #1598).